1. Field of the Invention
This invention relates generally to a method and apparatus for molding thermoplastic articles, and more particularly to a method and apparatus for filling a mold cavity at one station and then transferring the mold to a second station where pressure is applied to the thermoplastic material for curing.
2. The Prior Art
Various molding assemblies and methods have been disclosed and utilized in the prior art to form thermoplastic articles by injection molding. Typically, the methods include injecting thermoplastic material at relatively high pressure through a rather small sprue opening into a mold cavity and then thermally conditioning the material in the mold at a fixed position adjacent the injection assembly. Certain patents, such as U.S. Pat. No. 3,270,383, have disclosed the use of relatively large sprue openings in the injection process; while a larger group of patents have generally disclosed the concept of transferring thermoplastic material from an accumulator at relatively low injection pressures. Representative of this last group of patents are U.S. Pat. Nos. 3,231,656, 3,453,353, 3,280,236, 3,433,862, 3,947,203, 3,375,553, 3,196,198, and 3,092,440. In a related area, Patents such as U.S. Pat. Nos. 3,670,066 and 3,270,383 have disclosed the concept of applying pressure to thermoplastic material within a mold cavity, but only for the limited purpose of accounting for shrinkage.
This state of the art has resulted in several previously unsolved problems, as follows:
"Flashing" of the plastic material generally occurs between the mold and the accumulator, even in those methods involving low injection pressures because a high packing pressure is applied to the material in the mold before the material cools from a heated flowable condition.
Heat is transferred between the cooled mold and the adjacent heated accumulator, resulting in (a) cold slugs in the accumulator and (b) increased cycle time to properly cool the material in the mold.
Plastic material has been subject to degradation due to high injection pressures, high packing pressures, and unnecessarily prolonged cycle times.
Additionally, large hydraulic clamping equipment has been required to maintain the molds closed during both the injection cycle and the cooling cycle. As is common knowledge, such equipment is not only expensive to purchase and operate, but also requires frequent maintenance.